Seawalls as a defense against tsunamis are a solution with substantial flaws. Now a team from Stanford University has a twist on that age-old idea: building strategically positioned rows of hills along vulnerable stretches of coast.
The study advocating the construction of artificial hills was led by Jenny Suckale of Stanford, with scientists from the Naval Postgraduate School, the New Jersey Institute of Technology, MIT and Indonesia’s Ministry of Marine Affairs and Fisheries, and was published Monday in Proceedings of the National Academy of Sciences.
Tsunamis aren’t as rare as you might think. Relatively wee localized ones happen about twice a year on average worldwide. More rarely, every 15 years on average, tsunamis can impact a whole oceanic basin, though they’re much rarer in the Mediterranean.
People living in places prone to tsunami trouble likely know about it (assuming the relatives through the generations are on speaking terms). Going back thousands of years, some have been placing their faith in a specific defense against storm surges and other wave phenomena: the seawall.
Seawalls have their advantages, but they’re an imperfect solution to tsumanis for a host of reasons – the least being that if built properly, they’re an eyesore. If people come to seaside towns, they’re not there to look at a cement monstrosity. Moreover, seawalls are expensive to build, which is a second problem.
A third is that any seawall is only as strong as its weakest point. If the seawall collapses during the tsunami event, its rubble adds to the debris being forcefully transported over the land (note pictures of whole, not-small ships transported deep inland by mega-tsunamis).
A fourth problem is that in this era of global warming, sea levels are rising, and nobody can predict when or by how much. While climate change is definitely happening – there is zero doubt about that – its pace remains theoretical.
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“The coastline is far too dynamic to expect coastal construction plans to have longevity beyond a decadal scale, in any case,” observes Prof. Beverly Goodman of the University of Haifa’s marine geology department. Although she isn’t involved in this new study, Goodman is an expert on the archaeological record of tsunamis.
One thing that is not theoretical is that if a seawall collapses during a tsunami, “the consequences are life-shattering,” Suckale explains.
Which brings us to the fifth major problem with seawalls: they can create a false sense of security that can discourage swift evacuations, Suckale says.
A futile struggle 7,000 years ago
The Mediterranean isn’t one of the more tsunami-prone seas. Big ones may happen on average every 600 years, Goodman says, though one could get two mega-tsunamis in a short period of time, or there could be a thousand-year break. Just as earthquakes remain unpredictable, so do tsunamis, which are often but not exclusively caused by marine earthquakes or landslides.
Schooled by Hollywood and the like, we may think of tsunamis only in mega-terms, but relatively bitty local ones do occur. One recent example, Goodman says, is the tsunami that hit the Turkish town of Bodrum in July 2017 after a magnitude-6.3 earthquake on the seabed of the Aegean Sea near the Turkish coast.
Tsunamis were also reported in Izmit Bay after the August 1999 mega-quake at Kocaeli, Turkey. The waves weren’t very high, but the issue is less one of wave height than the sheer volume of water displacement. Storm waves in the Mediterranean can reach 14 meters (46 feet) in height, Goodman tells Haaretz. That would be terrifying to any beach-goer, but the water would stay in the sea. The problem with tsunamis is that the whole water column moves and as the tsunami reaches land, it encroaches with enormous force.
Last year, archaeologists reported finding the remnants of a monumental seawall built around 7,000 years ago in Neolithic northern Israel, which was likely built as the locals noticed sea level creep, or possibly after one of the piddling local tsunamis. (If it was a big one, there wouldn’t have been anybody left to remember it and tell the tale to following generations.) Whether erected to protect the coastal villages by today’s Carmel “mountain” range from the rising waters of the Mediterranean as the northern ice sheets melted, or whether it was built against huge waves, it was ultimately a failure, an international team of archaeologists deduced. They deduced that by finding it underwater.
That ancient example proves the point that if you can’t predict the future, you can’t protect against it. The Neolithic coastal village had originally been built at about 3 meters above sea level – and over generations, as the sea level rose, the water approached. We may assume that as the village slowly became engulfed, its residents noticed, abandoned their homes and moved inland.
In the case of tsumanis, one typically has very little time to scramble to higher ground. A buffer zone between the people and the raging sea, whether because of tsunami or storm surge or sea level change, is a good thing, Goodman explains: in the case of flat coastal land, the ocean can encroach kilometers inland. “If you live somewhere like the cliffs of Netanya, a tsunami will only affect the beach and cliff. If there’s no cliff like in the riverbed areas of the Kishon River and Yarkon River [in Tel Aviv] – we estimate it can go kilometers inland,” she says.
Head for the artificial hills
Japan, for one, had bitter experience with its enormous 40-foot-tall seawalls that protected it not when a big one hit. Coastal forests may help mitigate the impact of the tsunami, but how many coasts have forests any more?
So, heading for the hills is one option. If there aren't any, building preemptive hills in “tsunami mitigation parks” is another. And if they’re green rather than sand dunes, so much the better.
"The potential issue with sand dunes is that they might erode quite quickly when a tsunami impacts them," Suckale tells Haaretz. "That being said, one of the argument we make here is that the vegetation on top of the hills is not an essential component. (Some previous models have argued that they mitigate tsunami risks by elevating friction, but that’s a very small effect.) Vegetation could help with reducing erosion, but there also are other ways of doing that."
The problem, explain Suckale and the team, is that the designs of tsunami mitigation parks so far – featuring both walls and hills – tend to be guided more by aesthetics than science. This is where the new paper comes in, applying actual modeling to the plan.
Their modeling indicates that there’s no difference in mitigation if a tsunami hits one wall or a single row of hills. In the event of mega-tsunamis, both will fail anyway. But artificial hills can be custom-built and shaped based on the tsunami’s anticipated direction of movement and other site-specific parameters to maximize the amount of energy the hill reflects back. This is key, Suckale says: “Energy is really your main enemy.”
Wouldn't rows of hills be better than one row? "I think so," she answers. "There is definitely still a lot of room for improvement in the design of the parks and your suggestion sounds like a good one to me (but I haven’t quantified it specifically quite yet). We are currently exploring these kinds of setups in our ongoing work."
Goodman finds the concept intriguing and logical, she tells Haaretz. “There’s been a lot of awareness that natural coastal buffers like mangroves and even coral reefs and other things along the coastline can attenuate tsunami damage” – at least when they’re not disturbed but are maintained in pristine shape, she says. The buffers will be damaged by the tsunami, no question, but they can slow it down and attenuate its impact.
The concept of artificial hills would likely mimic what happens when tsunamis slam into coastal dunes, Goodman explains. A really big tsunami will flatten the “frontal” dunes it hits, but will slow down and “calm” as it hits the next, and so on. And while the news team may pour scorn on the aesthetic as muse, they do stress that rolling green hills are more attractive than a great hulking mega-wall that’s fated to fail anyway at some point.
Haaretz wonders about the strength of a great wall versus a hill in the case of a quake causing a tsunami. The hill might rock back and forth, but it wouldn’t crack and collapse. But is building hills or rows of hills even feasible over huge vulnerable coastlines like around the Pacific Rim?
"he goal of this paper is to put more 'options on the table," Suckale answers. "Finding the best one out of these depends on a variety of factors, including community vulnerability, the dependence on coastal livelihoods like fisheries, ecosystem health, cultural preference and money. In this project, we worked primarily with Indonesia. They have huge coastlines with a very high tsunami risk, but extensive walls are not an option for a number of reasons. So yes, I would tend to think that this is an option worth considering. Whether it’s the optimal solution for any given coastline is for the local communities to decide."